LiFePO4 - The future for off-grid battery banks?

[billvon]

I am trialling an idea that I've kicked around for the last 6 mths or more, running the solar direct and load shedding into the water heater if needed. The idea revolves around the MPPV of 12v panels being between 17.5v cold and 15v hot and a 5 Li cell pack having a capable voltage range of 14v (2.8v per cell) to 20V (4v per cell) The 20v end is past the point the panels make any current so that end is reasonably safe although high cell cut (HVC) is still need as a back up in case the cells go out of balance and a low voltage cut (LVC) to protect the cells from a very deep discharge. This 5 cell pack is used to charge the operating house power 4 cell pack via a solid state relay and heavy cabling.

Yikes. A few issues there:

LiFePO4 is more rugged than LiCo, but are nowhere near as rugged as lead acid. A bang-bang (PWM) controller used with a diversion load is going to tend to hurt them since you see momentary overvoltages with them. Also they're not very tolerant of voltages over about 3.8 volts.

Also you can't rely on "the 20v end is past the point the panels make any current." The time you run into that is on a cold day when you're not there and the panel has all day to take the battery up to 24V. You absolutely need a good charge controller - and for a lithium chemistry it's probably going to have to be switchmode.

Sort of a really big nearly super capacitor set up.

Batteries are much more finicky than supercaps. Supercaps are indeed a lot more rugged, provided they are balanced and kept under their voltage limits.

[T1 Terry]

These are the lithium ferrous phosphate cells (LiFeP04) they are damaged if pulled down below 2v and held there apparently as the electrolyte heating happens at both ends of the voltage range, hence the cell case swelling. I will try recompressing the cell and test how much capacity I've lost, I did a similar thing at the high voltage end to anoter 2 cells so I'll build a 12v battery out of them with one good cell and measure capacity down to 2.8v per cell and see if the one good cell is still at a high voltage at the end and if all 90ah are still there. Maybe (hopefully) it only effects their charge and discharge rate, that's not as critical in a house power set up as it is in an EV set up.

T1 Terry

[inetdog]

It has worked well for a couple of days but unfortunately I got side tracked on helping a mate with is project and didn't have an operational LVC, one cell hit 0.456v by the time I got home to check it. Over charging isn't the only thing that causes these cells to bulge, hopefully I'll be able to recover it as it was a new cell.

T1 Terry

If this is like the other common Li chemistries, as found in Li-ion batteries for computers, tools, etc. There is a point (voltage) in the discharge cycle at which the chemical reaction shifts from reversible to irreversible. When you go past this point, the cell becomes useless.

Many people think that the reason a tool using an Li battery stops suddenly without warning at the end of its charge is that the discharge curve drops suddenly. That is not the case. Instead the protective circuitry in the battery pack has cut off the output to protect the cells. That is also why, unlike NiCad or NiMh packs it does not recover at least a little after being left idle.

If one cell actually got to .456 volts, it is gone forever.

[T1 Terry]

Haven't been on here for quite while, how is your system holding up Steve? My 720Ah Winston cell Li system has been struggling through the depths of winter, hovered around 20% down to 0% each day for around a mth but they are clambering back now. A very large gum tree in our front yard hasn't helped things at all with the sun hiding behind it from around 11am to 3.30pm but now it's starting to climb a little higher in the sky things are looking better.
I am trialling an idea that I've kicked around for the last 6 mths or more, running the solar direct and load shedding into the water heater if needed. The idea revolves around the MPPV of 12v panels being between 17.5v cold and 15v hot and a 5 Li cell pack having a capable voltage range of 14v (2.8v per cell) to 20V (4v per cell) The 20v end is past the point the panels make any current so that end is reasonably safe although high cell cut (HVC) is still need as a back up in case the cells go out of balance and a low voltage cut (LVC) to protect the cells from a very deep discharge. This 5 cell pack is used to charge the operating house power 4 cell pack via a solid state relay and heavy cabling.
It has worked well for a couple of days but unfortunately I got side tracked on helping a mate with is project and didn't have an operational LVC, one cell hit 0.456v by the time I got home to check it. Over charging isn't the only thing that causes these cells to bulge, hopefully I'll be able to recover it as it was a new cell.
Basically, the idea is to store the added available solar output rather than loosing it to voltage mismatch through PWM control or heat through MPPT control. The price of a truely worthwhile MPPT unit buys a lot of batteries and simple on/off circuits to drive solid state relays.
The extra Wh harvested this way is quite impressive as is how much quicker the panels produce an input to the batteries compared to MPPT, well compared to the 4 units I've trialled anyway, no one has given me an Outback MX 80 to test, yet The cost of one of those units buys over 250Ah on lithium @ 12v so it's worth the effort if it works as the added storage keeps the main battery bank full well after sun down. Sort of a really big nearly super capacitor set up.
Once I've ironed the bug out and have a better operating control system I'll post more on it, or if it's a complete diaster I'll post that as well to save anyone else going down that path.

T1 Terry

[Coolrio]

Greetings from New Zealand too

Hi Steve g:

Am planning to build an off grid earthship in Hawkes Bay, New Zealand. Would appreciate connecting with you to find your experiences with the LifePO4 cells in your solar system.

Warm regards,

Riaz

[john p]

Sorry im not getting the connection? The guy was asking about a home install. I dont think his house in Nigeria is going to be in the same league as large industrial installation requirements are.??

[billvon]

BILLVON..If I was using 3600kwh a month I would be seriously worried.

I installed an off-grid solar power system at an airport that used about 5000kwhr a month in the summer - a few air conditioners, two refrigerators, five structures with CF lighting, aircraft radios, PA etc. The solar power system (about 3kW STC DC) kept the system running when no one was there, basically just the answering machines and the refrigerators. During operations they ran a 10kW gasoline generator, and the two inverters (11kW worth) supplied the starting surges for the A/C. Battery bank was an 8s4p bank of T-105s for a total storage of about 42kWh.

They kept growing and eventually got sick of maintaining the batteries all the time, so bought two 50kW propane generators. Now they just run one all the time, and keep the other as a backup. (And since they have a few military contracts now, they can afford the fuel and service.)

[john p]

BILLVON..If I was using 3600kwh a month I would be seriously worried.

[billvon]

I am new to solar and I am not that technical by any stretch. I have read some of your messages on this thread and it sounds like LiFePO4 has a higher initial cost but lower maintenance than lead acid batteries. I am currently in Nigeria: a country that has a lot of power outtages. I am looking for good home energy system to compliment our spastic grid here. I estimate a house load of 5kva. Please advise on the most cost effective brand of solar panels, inverter and battery pack to buy and their respective costs.

5kva is a power, not an energy, requirement. You have to measure the total energy used by the house over the course of a day. If it's an average, then you use about 120kwhr/day. If you get 5 hours of sun a day that's a 24kW system.

The absolute minimum price for the solar panels alone is about $1 a watt - so we are talking $24,000 for just the panels. And that's cheap chinese panels.

Is this within your price range?

[Bigdo]

Home Energy Back up System

Hi Everyone,

I am new to solar and I am not that technical by any stretch. I have read some of your messages on this thread and it sounds like LiFePO4 has a higher initial cost but lower maintenance than lead acid batteries. I am currently in Nigeria: a country that has a lot of power outtages. I am looking for good home energy system to compliment our spastic grid here. I estimate a house load of 5kva. Please advise on the most cost effective brand of solar panels, inverter and battery pack to buy and their respective costs. Sorry moderator but I will like the names of companies and location so I can contact them for quotes to import. There are a few solar engineers I can employ to install for me. Deep cycle batteries and generators are very popular here and it has to to do with their relative cheapness. Thank you in advance.

[TommyL]

This is all Awesome Information! This is awesome, but I'd like to do this on a small scale and expand. I've built my own house.
Yes, even pulled my own electrical permit. I will be having my final inspection this summer, so I will contact the Electrical Authority
and see if I require a different permit or if existing will cover it.

We have the new digital meter here. I'd like to find out more about that might be a benefit or not.

[billvon]

Any thoughts?

Tommy L sends....

If you have grid available it is always better (=easier, cheaper, more efficient, cleaner) to use the grid as your battery. Store energy in it during the day, run on the grid at night. It's even good for the grid - you are feeding in during times of highest use and drawing from it during times of least use. If you live in an area that has frequent power outages, then float your batteries (they last longer that way) and use them only for backup.

You can get about 500 cycles out of your run of the mill batteries, probably 2000 out of your LiFePO4 batteries. For a typical house you're talking at least 12kwhr per day, so you would need 12,000 / (8ah * 3.2V) = 470 batteries, for a cost of around $10,000, for even one day of storage. They would have to be replaced every 5 years. So that's an additional $166 a month you'd have to pay to cycle through your batteries.

[russ]

The grid tie system uses the grid as an immense battery in the case Sunking mentioned

[Sunking]

Good day all.....
I'm not very good at this forum stuff, but I guess it's correct to stay on topic.

I will use Headway 38120HP (Red Cells) for an off grid system.

I've seen people using this in type of system while on Grid safely with a
Generator style pony panel. So you could collect sun all day and store into batteries,
run the house clocks and low consumption items while no one is home. Continue to
use during the evening until more consumption is required, then switch over to Grid power.

This way you get the best of both worlds, a back up without fuel and using solar to supplement

Any thoughts?

Tommy L sends....

Tommy what you are talking about is a hybrid system. The problem is it doesn't work like you think. Battery power is extremely expensive, on the order of 10 to 20 times more than commercial power. It is pretty wasteful to spend that when you are already connected to the grid.

Better solution is plain ole grid tied system, where all that power during the day goes right out on the grid building up your credits, so when you come home you get to burn your credits and net 0 usage. If you want emergency power, install a generator using either LPG, NG, or diesel fuel. It is a lot less expensive, no limitations on power, and much better on the environment. Its a win-win no brainer.

[TommyL]

Good day all.....
I'm not very good at this forum stuff, but I guess it's correct to stay on topic.

I will use Headway 38120HP (Red Cells) for an off grid system.

I've seen people using this in type of system while on Grid safely with a
Generator style pony panel. So you could collect sun all day and store into batteries,
run the house clocks and low consumption items while no one is home. Continue to
use during the evening until more consumption is required, then switch over to Grid power.

This way you get the best of both worlds, a back up without fuel and using solar to supplement

Any thoughts?

Tommy L sends....